Draining a Pressure Vessel

There are lots of liquid collection points in a compressed air treatment system. Moisture separators. Coalescing filters. Wet receiver tanks. Dryers. All of these must be drained.

It seems like an easy task, right? It should be a no brainer. After all, how could draining a tank take any thought or skill, especially when it is under pressure? Yet I’m always surprised by how often I get a troubleshooting call where the problem is traced back to a flawed drain system. A bad drain design turns the compressed air system into a sprinkler system. All that water wreaks havoc and ends up costing a lot of money for repairs and re-work.

A flooded air system leads to what we call Death by Drowning. That’s a metaphor, guys.

Here are few things you can do to improve the draining system.

1. Drain pipes must be pitched downwards from whatever you use to drain the vessel. Otherwise, standing liquid corrodes metal pipes or can freeze. A pitch of a 1/4” per foot of pipe is a good guideline.

2. Make sure that the drain pipe has a vent. Otherwise, you may accidentally pressurize the building’s drain system and have geysers all over the place. Humorous to some, but not safe or productive. It may even damage piping.

3. Inexpensive float drains fail. Not if- When. Here’s a secret: the little plastic nub that is sold as an “automatic drain valve” at the base of that moisture separator, it’s not worth the $20 bucks you paid. Get a direct acting durable auto drain.

4. Don’t try to save a buck by attempting to drain multiple vessels with a single auto drain. This cheap trick has been soaking air systems for years. I can’t emphasize this enough. Put an automatic drain on each drain point. When you try to drain multiple vessels with one drain, you end up backing liquid up into the last of the vessels. This is because the pressure in this vessel is slightly lower than the others. Even a pressure differential of two PSIG will push water up almost five feet. Take a look at the quick illustration below. This shows what you SHOULD NOT DO.

Now here’s an illustration of what you have to do.  MDVs are shown, but any of the auto drains will work.  The pressure differences don’t matter now, because the drain lines are not connected.